This invention relates generally to a multiplexing system, and, more particularly, to a multiplexer/demultiplexer which incorporates therein a holographically formed reflective grating.
The need frequently arises for communications or reconnaissance systems which simultaneously convey multiple messages from a large number of information sources in one location to a large number of users at another location. Multiplexing systems economically meet this need by combining the messages from several information sources, which are then transmitted as a composite group over a single transmission facility, with provision at the receiver for separation (demultiplexing) back to the individual messages. Since only one transmission facility is needed instead of many, one advantage of multichannel operation is a lessening of the total quantity of necessary equipment. Each of the individual streams of information that form a multiplexed group are often denoted as a channel.
There are generally two generic forms of multiplexing systems. These are wavelength or frequency division multiplexing and time division multiplexing. Frequency division multiplexing is directly applicable to continuous wave form (analog) sources, in essence involving stacking side by side in frequency several information channels so as to form a composite signal. The composite frequency-multiplexed signal is then used to modulate a carrier in some conventional manner. Recovery of the individual messages after reception and demodulation is accomplished by bandpass filtering and frequency selection of the channels.
Time division multiplexing is a logical extension of pulse modulation and involves interleaving in time the narrow pulses of several ordinary pulse modulation signals and thus form one composite pulse transmission system. Separation of the time multiplexed pulse streams at the receiver is accomplished by gating appropriate pulses into individual channel filters.
A third technique, phase multiplexing, is possible but appears less practical than either frequency division multiplexing or time division multiplexing.
In recent years with the development and implementation of fiber optic technology into practical transmission systems a great deal of attention has been given to the multiple carrier technique referred to as wavelength division multiplexing (WDM). This technique, which is the optical equivalent of frequency division multiplexing techniques employed in RF coaxial transmission networks, can be used to increase the information transfer capacity of the medium. In the wavelength division multiplexing technique each discrete data channel is modulated onto an optical carrier of a fixed wavelength. Each of the individual carriers are then superimposed onto the optical transmission medium. At the optical receiver the individual carriers must be reestablished by separating the composite carrier into its individual wavelength components.
One of the parameters that encourages the development of wavelength division multiplexing is the inherent narrow spectral characteristics of optical sources and the relatively wide spectral bandwidth of an optical fiber. To achieve the desired optical separation and make wavelength division multiplexing practical, a series of optical techniques have been examined. Some of these techniques employ prisms, blazed plane reflection gratings, simple thick reflection gratings, multiple thick reflection gratings, simple thick transmission gratings and dichromatic coatings.
Unfortunately, size and expense are critical factors when providing optical multiplexing systems. To date, however, multiplexing systems have been either unreliable in complete separation of the individual wavelength components or are of such size and cost so as to provide an overall system which is incompatible with the desired small size and economic requirements. Consequently, a need arises for a small, lightweight, inexpensive device which can be effectively used as both a multiplexer and demultiplexer and yet be compatible with the inherent narrow spectral characteristics of today's optical sources.